In an exhaust system of an internal combustion engine which performs lean combustion, such as in a diesel engine, a NOX catalyst device is arranged to purify NOX in the exhaust gas. One known NOX catalyst device produces ammonia by hydrolysis of urea supplied with the exhaust gas and selectively reduces NOX in the exhaust gas by using of ammonia NH3 as a reducing material.
An amount of urea per a unit time just sufficient to reduce an amount of NOX per a unit time flowing into such a selectively reduction type NOX catalyst device indicates “an amount of urea of an equivalence ratio of 1”, and it is desirable that an amount of urea of the equivalence ratio of 1 is always supplied into the NOX catalyst device. However, when an amount of NOX per a unit time flowing into the NOX catalyst device changes by a change of the engine operation, it is difficult that an amount of urea of the equivalence ratio of 1 is supplied to the NOX catalyst device to follow the change of an amount of NOX per a unit time flowing into the NOX catalyst device.
Accordingly, an exhaust purification system of an internal combustion engine comprising the selectively reducing type NOX catalyst device is known, in which urea holding ability is provided to hold a certain amount of urea. When an amount of NOX per a unit time flowing into the NOX catalyst device increases to follow a change of the engine operation, even if an amount of urea of the equivalence ratio under 1 is supplied, it is intended that ammonia can be produced from the held urea so as to compensate for a shortage in ammonia, such that NOX, in the exhaust gas can be satisfactorily reduced. On the other hand, when an amount of NOX per a unit time flowing into the NOX catalyst device decreases to follow a change of the engine operation, even if an amount of urea of the equivalence ratio over 1 is supplied, it is intended that extra urea can be held by the urea holding ability to restrain the ammonia flowing out.
In the above exhaust purification system, the amount of urea that can be held by the urea holding ability changes according to the temperature of the NOX catalyst device. Accordingly, it is suggested that a target amount of held urea is set for each temperature of the NOX catalyst device and to realize the target amount of held urea for each temperature of the NOX catalyst device, urea supplying is stopped when it is estimated that an amount of urea held in the NOX catalyst device is larger than the target amount of held urea and an amount of urea of the equivalence ratio over 1 is supplied when it is estimated that an amount of urea held in the NOX catalyst device is smaller than the target amount of held urea (for example, refer to Japanese Unexamined Patent Publication Nos. 2003-293737 and 2005-201218).